Effects Of Irrigation And Nitrogen Application Rates On Hayyield,Water Use Efficiency Of Alfalfa And Fertilizer Nitrogen Loss

In the Shiyang River Basin, the environmental deterioration caused by excessive nitrogen (N) and irrigation application in agricultural ecosystems has received much attention in recent years. A combination of careful irrigation and N management is needed to improve N uptake efficiency and to minimize fertilizer N loss in the arid oasis agricultural area of Northwest China. A field experiment was conducted to investigate the effects of different irrigation and N application rates on hay yield, water use efficiency (WUE), plant N uptake, soil nitrate N (NO3--N) distribution in soil profile and fertilizer N loss in the oasis region of Shiyang River Basin, Gansu province. The experiment followed a completely randomized split-plot design, taking flood irrigation (conventional irrigation (327mm),20%water-saving irrigation (261mm),40%water-saving irrigation (196mm)) and N fertilizer application rates (0,40,80,120kg N hm-2) as the main-plot and split-plot, respectively. The results were followed:The hay yield of alfalfa increased with the increase of amount of irrigation, and the water use efficiency (WUE) of alfalfa first increased then decreased with the increase of amount of irrigation. The hay yield of alfalfa (15905kg hm-2) and WUE (28.62kg mm-1hm-2) reached the highest when the N supply rate reached40kg N hm"2. Considering from economic return and WUE, the treatment of20%water-saving irrigation combined with N supply level at rate of40kg N·hm-2was best choice for the second year alfalfa plating in Hexi oasis of Gansu Province.After analyzing the differences of N content in leaves and stems between different irrigation rates, and between different N application rates, the differences between treatments were not significant. Because the high water and N supply rate increased the N transferring from leaves and stems to hay yields, and the plant N uptake increased with water and N supply levels. Compared with no N application, the increases of the plant N uptake in the120,80and40kg N ha-1were16%,16%and20%, respectively, and the increases of the plant N uptake in the traditional irrigation were13%and14%compared with20%saving irrigation and40%saving irrigation, respectively.The nodule bacteria was formed as alfalfa roots growing. And this N fixing system could uptake N from atmosphere. The NO3--N accumulation in the200cm soil profiles at harvesting stage (35-77kg hm-2) was more than before the sowing stage (20-25kg hm-2). Seasonal changes in soil NO3--N ranged from206kg N ha-1to262kg N ha-1. It was concluded that N mineralization and N fixing was higher than fertilizer N losses during the alfalfa growing seasons.The amount of residual NO3--N in soil profiles increased with N supply levels at the third alfalfa harvesting stage. The increases of fertilizer N loss in120kg N ha-1were21%,77%and122%, compared with80,40and0kg N ha-1, respectively. The amount of residual NO3--N in soil profiles decreased with the amount of irrigation. The decrease of residual NO3--N in soil profiles in traditional irrigation were14%and37%, compared with20%saving irrigation and40%saving irrigation, respectively. Nitrogen and irrigation supply increased nitrate leaching and N2O emission, and resulted in a high fertilizer N loss. Compared with no N application, the increases of fertilizer N loss, in120,80and40kg N ha-1, were37,10and-15kg N ha-1, respectively. Compared with40%saving irrigation, the increases of fertilizer N loss, in traditional irrigation and in20%saving irrigation, were9and19kg N ha-1, respectively.Although the differences of total soil N content were changing with soil layers and with treatments, but total soil N content in40kg N ha-1was higher than those in120,80and0kg N ha-1in many cases. The optimum N rate (40kg N ha-1) could promote nodule bacteria forming and growing, and increase hay yields and total N content in soil. Irrigation supply increased the total N content in soil. Compared with40%saving irrigation and20%saving irrigation, the increases of the total N content in soil, in traditional irrigation were15%and3%, respectively.